1. Field of the Invention
This invention relates to circuit interrupters and, more particularly, to medium voltage vacuum circuit breakers.
2. Background Information
Circuit interrupters, such as circuit breakers, provide protection for electrical systems from electrical fault conditions such as, for example, current overloads, short circuits and abnormal voltage conditions. Typically, circuit breakers include a spring powered operating mechanism, which opens electrical contacts to interrupt the current through the conductors of the electrical system in response to abnormal conditions.
Vacuum circuit breakers employ separable main contacts disposed within an insulating housing. Generally, one of the contacts is fixed relative to both the housing and to an external electrical conductor, which is interconnected with the protected circuit. The other contact is movable. The movable contact assembly usually comprises a stem of circular cross-section. At one end, the movable contact is enclosed within a vacuum chamber and, at the other end, a driving mechanism is external to the vacuum chamber. An operating rod assembly comprising a push rod, which is fastened to the end of the stem opposite the movable contact, and the driving mechanism provide the motive force to move the movable contact into or out of engagement with the fixed contact. The operating rod assembly is operatively connected to a latchable operating mechanism, which is responsive to an abnormal current condition. When an abnormal condition is reached, the latchable operating mechanism becomes unlatched, which causes the push rod to move to the open position.
Vacuum circuit interrupters are typically used, for instance, to reliably interrupt medium voltage AC currents and, also, high voltage AC currents of several thousands of amperes or more.
Medium voltage circuit interrupters operate at voltages of from about 1 to 38 kV. Such circuit interrupters, being relatively large and heavy, are mounted on trucks for insertion into and removal from metal enclosures or cabinets in which they are housed. As the circuit interrupter rolls fully into position within the enclosure, contact fingers engage stabs, which connect the circuit interrupter to line and load conductors. Withdrawal of the truck disconnects the circuit interrupter from all conductors, thereby assuring a safe condition for maintenance or removal.
Medium voltage circuit interrupters employing vacuum interrupters typically have a front low voltage portion just to the rear of a front panel, which typically includes the breaker controls. The low voltage portion is electrically insulated, through the use of standoffs, from high voltage portions, which include the vacuum interrupters. The standoffs permit sufficient spacing of the conducting components in order that air within the housing can be an effective insulator to isolate the low voltage portions from the high voltage portions. However, this spacing also contributes to the size of the units.
More recently, attempts have been made to reduce the size of these units by hermetically sealing them and filling them with a gaseous insulator having a higher dielectric capacity than air.
Interruption of a medium/high voltage circuit advantageously requires a current interruption device that rapidly brings the current to zero upon the occurrence of a line fault. A “high” voltage fuse is of a type employed in electrical power distribution circuits typically carrying voltages of about 1 to 38 kV. Line faults at these high energy levels can cause extensive damage to circuit components and devices connected to the circuit, or to conductors and various other portions of the electrical energy distribution system. To minimize potential damage, fuses are employed with the intent to interrupt current flow quickly, following the onset of fault conditions involving high current loading, such as a short circuit or overload faults.
There is room for improvement in medium voltage vacuum circuit interrupters.